1. Field of the Invention
The present invention relates to radiation curable coating compositions comprising unsaturated polyester resins containing at least two unsaturated groups, a reactive monomer or mixture of monomers, photoinitiator, and between about 4 wt. % to about 35 wt. % of an organosoluble, film forming, cellulose derivative.
2. Description of the Related Art
Solutions of unsaturated polyester resins in styrene or vinyl toluene are commonly used in radiation curing. These resins are particularly prone to oxygen inhibition of cure because of their slower rate of polymerization. In appli-cations where rapid and thorough surface cure is required, the surface must be physically protected from oxygen. Methods of oxygen protection include the use of a photo initiator which scavenges dissolved oxygen during irradiation and blanketing the coated substrate with an inert gas such as nitrogen. The present invention replaces the toxic styrene monomer of typical unsaturated polyester resin systems with reactive monomers which reduce the oxygen inhibition of these systems by increasing the cure or copolymerization rate.
It is known that under some circumstances, alternating copolymers may form faster than homopolymers. This is especially true if the electronic characteristics of the monomers are complimentary such that one is electron rich, and the other is electron poor. Under these conditions there is a polar character to the radical intermediates which propagate the growing chains. There is a stronger driving force for them to react with the monomer of opposite polar character than with their own monomer. The reactive monomers utilized by the present invention have a complimentary electron rich character compared to the maleate and fumarate functionality present in the unsaturated polyester resin.
A British patent application, GB 2,159,524 dated Dec. 14, 1985, describes the thermal solution copolymerization of cellulose graft copolymers containing maleic side groups with monofunctional vinyl monomers to form soluble cellulose ester graft copolymers for use as thermoplastic resins in thermoset coating compositions. Also disclosed is the use of these cellulose graft maleic copolymers in thermoset coatings with hydroxyl group crosslinking agents such as amino resins and polyisocyanates. The patent application does not disclose the use of vinyl reactive monomers in conjunction with cellulose derivatives, photoinitiator, and unsaturated polyester resin to produce radiation cured coatings.
The present invention seeks to overcome many of the limitations found in the radiation curable systems currently in use. The objectives of the present invention are reduced toxicity, reduced oxygen inhibition to radiation-induced cure, fast cure speed, and low cure temperatures.